This invention relates to franking machines and in particular to a method and apparatus for printing a franking impression on mail items such as envelopes and labels for attachment to larger items.
Mail items are franked by a franking machine which is arranged to print a franking impression on the mail items to indicate that a postal value has been charged for the transmission of that item. With small mail items comprising an envelope, the franking impression is usually applied directly to the envelope. However with larger items such as parcels and packets, too large to pass through the printing device of the franking machine, the franking impression cannot be applied directly to the item and instead is applied to a label which is subsequently adhered to the parcel or packet.
Franking machines include a printing device to print the franking impression and other material, such as an advertising slogan, may also be printed on the envelope or label by the printing device during the passage of the envelope or label through the franking machine. The printing has usually been effected by means of type characters which engage an inked roller and then the surface of the mail item or alternatively press an inked ribbon against the surface of the mail item. The franking impression comprises a pattern determined by the postal authority and includes the value of postage and the date of franking. The franking impression may include further data such as an identifier for the franking machine and coded data for checking the validity of the franking. Commonly the printer has a printing drum which is rotated to bring type carried by it into engagement with an inked roller and thereafter into printing engagement with a mail item. The drum may carry fixed type to print invariable data such as the officially authorised fixed pattern of the franking impression together with type characters which can be selected for the printing of the variable data such as the date and value of postage charged. The type characters for printing variable data are usually carried on rotatable type wheels which can be turned to bring selected characters into an operative printing position. Such printers require complex mechanisms for setting the type wheels and also electro-mechanical transducers for generating electrical signals, representing the postal value to which the type wheels are set, to provide electrical signals for input to electronic circuits of the postal value selected to be printed to enable the circuits to carry out accounting functions related to the use of the franking machine.
It has also been proposed to use other forms of printer such as thermal dot matrix printers which use a thermal transfer process for the transfer of ink from a transfer ribbon to the surface of the mail item. Such printers have a print head with a plurality of selectively heatable print elements which bear against a rear face of the transfer ribbon. The front face of the ribbon has a layer of inked material and the mail item is fed with its surface in contact with the front face of the ribbon. The layer of inked material of the ribbon is melted selectively by heating of the heatable elements of the print head. As a result the ink in the vicinity of the heated elements is transferred to the surface of a mail item contacting the ribbon. Thermal dot matrix printers have an advantage over the mechanical printers used in earlier franking machines in that they are electrically operated and hence do not require the provision of complex mechanical assemblies for setting mechanical printing devices to the values required to be printed. Furthermore because the postage value printed by the thermal dot matrix print head is determined directly by print drive signals originating from the accounting and control circuits there is no requirement to provide mechanical to electrical transducers to input electrical signals from the printer to the electronic circuits. However both forms of printer require the provision of an inked roller or inked ribbon. In use, ink is removed from the inked roller and after a relatively large number of passes of the type past the printing position the ink remaining in the roller is reduced to a level at which the printing would be too feint to be acceptable. Similarly, with thermal transfer ribbons, the ink is removed from the ribbon by the heated printing elements and, if the ribbon is a single strike ribbon, the ribbon is not able to be re-used. If a multi-strike ribbon is used, the ink will be reduced to a level at which the printing is unacceptable after a relatively small number of passes of the ribbon. Accordingly at intervals during the use of a franking machine it becomes necessary to interrupt use of the machine in order to replace the used ink roller or ribbon with a correspondingly fully charged roller or ribbon. Such interruptions are inconvenient to the user of the machine particularly if it should occur during a run of franking operations.